The dose response adjusted for these variables is shown inFig

The dose response adjusted for these variables is shown inFig. TheETV6-NTRK3prevalence in post-Chernobyl PTCs was associated with increasing131I dose, albeit at borderline significance (p=0.126). The group of rearrangement-positive PTCs (ETV6-NTRK3,RET/PTC,PAX8-PPAR) was associated with significantly higher dose response compared to the group of Cyclopropavir PTCs with point mutations (BRAF,RAS) (p<0.001). In vitro exposure of human thyroid cells to 1 1 Gy of131I and -radiation resulted in the formation ofETV6-NTRK3with a rate of 7.9 106and 3.0 106cells, respectively. == Conclusions == We report here the occurrence ofETV6-NTRK3rearrangements in thyroid cancer and show that this rearrangement is significantly more common in tumors associated with exposure to131I and has a borderline significant dose response. Moreover,ETV6-NTRK3can be directly induced in thyroid cells by ionizing radiationin vitroand therefore may represent a novel mechanism of radiation-induced carcinogenesis. Keywords:thyroid cancer, radiation, chromosomal rearrangements, NTRK3, Chernobyl == INTRODUCTION == Thyroid cancer is the most common type of endocrine malignancy, and its incidence has been steadily growing in the U.S. and many other countries during the last four decades.1Exposure to ionizing radiation during childhood is a well-established risk factor for thyroid cancer. The increased risk of thyroid cancer after radiation exposure was first suggested in 1950 in infants who received external beam radiation for enlarged thymus.2This has been later confirmed in multiple studies Cyclopropavir of patients exposed to environmental or medical radiation, including X-ray or -radiation as wells as radioiodines, mainly iodine-131 (131I).3In the decades following the 1986 Chernobyl accident, the surrounding geographic area experienced a marked increase in incidence of thyroid cancer among those who were children or young adults at the time of the accident.4The post-Chernobyl case-control and cohort studies confirmed previous observations that papillary thyroid carcinoma (PTC) is the predominant type of thyroid cancer associated with radiation exposure,46and that the risk for PTC following131I exposure increases with dose, with the magnitude of the increase comparable to that following external radiation.710 Activating mutations in the mitogen activated CALCR protein kinase (MAPK) signaling cascade are common in thyroid cancer and believed to be essential for tumorigenesis.11The most common events include point mutations in theBRAFandRASgenes as well as chromosomal rearrangements involving theRETgene, known asRET/PTC. However, the mutational mechanisms leading to MAPK activation in sporadic and radiation-related PTCs appear to be different. Whereas in sporadic tumors, point mutations inBRAFandRASgenes are by far most common (60% of all PTCs), in post-Chernobyl or post-radiotherapy PTCs, 5080% of tumors typically harbor chromosomal rearrangements of theRETgene known asRET/PTCwhereas point mutations are rare.1214Other chromosomal rearrangements, such asAKAP9-BRAFand those involving theNTRK1andPPAR genes, are also more frequently found in radiation-associated PTCs.15,16However, a significant proportion of radiation-associated tumors harbor none of the known mutations, suggesting that other, unknown genetic events may occur in these tumors. In our recent study, we performed genotypic analysis of 62 PTCs from a well-characterized cohort of Ukrainian individuals (UkrAm) who received 0.0088.6 Gy of131I to the thyroid after the Chernobyl accident.15The study confirmed theRET/PTCrearrangements as the most common genetic event in these tumors and found different trends with dose in PTCs harboring chromosomal rearrangements and point mutations. However, 40% of these tumors had none of the known genetic events and were further analyzed in this study using the RNA-Seq analysis to discover novel genetic events that might occur in radiation-related thyroid cancer. This analysis revealed theETV6-NTRK3chromosomal rearrangement, previously unknown to occur in thyroid cancer, is a common genetic event in Cyclopropavir radiation-related but not in sporadic thyroid cancer, and showed thatETV6-NTRK3can be directly induced in human thyroid cells by ionizing radiationin vitro. == MATERIALS AND METHODS == == Study cases and samples == The study was approved by the University of Pittsburgh, National Cancer Institute, and Institute of Endocrinology and Metabolism (IEM) (Kyiv, Ukraine) Institutional Review Boards. The cases of radiation-associated PTCs were diagnosed among individuals from the UkrAm study who were younger than 18 years old at the time of the Chernobyl accident.17Individual radioactivity measurements in thyroid gland were performed within the two months following the accident and individual131I thyroid doses were estimated based on these measurements, interview data concerning dietary and lifestyle habits, and environmental transfer models.18,19PTC was diagnosed in 104 individuals (age at surgery: range, 1432 years; mean, 22.75.1 years) between 1998 and 2008 at the Laboratory of Morphology of Endocrine System of the IEM, after four sequential screenings.20Pathologic diagnoses were reviewed by the International Pathology Panel of the Chernobyl Tissue Bank (CTB). Frozen tissue samples were available for 75 cases of PTCs. For 74 PTCs DNA and/or RNA were extracted at IEM or Imperial College (London, UK) and received through the CTB. Four cases exposed to131Iin.